FIG. 8 is a simplified block diagram showing a portion of an electronic system 40 including two electronic devices 41 and 42 that operate using different operating voltages VDD and VHV, where VHV is higher (or lower) than VDD. Due to the different operating voltages, first electronic device 41 generates data signals having a voltage swing between GND and VDD (e.g., logic “1” data values are represented by 1V signal levels, and logic “0” data values are represented by 0V signal levels), whereas second electronic device 42 generates data signals having a voltage swing between GND and VHV (e.g., logic “1” data values are represented by 2.5V signal levels, and logic “0” data values are represented by 0V signal levels). Those skilled in the art recognize that data signals generated by device 41 typically cannot be utilized by device 42, and vice versa, due to the different voltages representing logic “1” data values.
In general, voltage level shifters are electronic circuits that facilitate communication between two electronic devices by converting “input” signals generated at the operating voltage of the first device to “output” signals that match the operating voltage of the second device. By way of example, as indicated in FIG. 8, a level shifter 50 is included in system 40 between devices 41 and 42, receives “input” data signals (DATA1) from device 41 having VDD/0V voltage levels, and generates “output” data signals (DATA2) having VHV and 0V signal levels, thereby facilitating the transfer of data signals from device 41 to device 42. More specifically, an “input” data signal DATA1 having the digital (binary) value 1010 is represented by a series of voltage signals respectively having the signal levels VDD, 0V, VDD and 0V, respectively. Level shifter 50 converts this “input” data signal to an “output” data signal DATA2 having the same digital (binary) value 1010, but having the voltage levels VHV, 0V, VHV and 0V, respectively, and transmits this “output” data signal to device 42. In this way, the “input” data signals generated by device 41 is converted to form “output” signals whose voltage levels match the operating voltage device 42.
In addition to converting “input” signals to match the operating voltage of the second device, level shifters often include slew rate control circuitry that allows selective control of the “output” data signal slew rate. Referring again to FIG. 8, according to one approach, level shifter 50 receives a slew rate control current T and includes circuitry (not shown) that adjusts the slew rate of “output” data signal DATA2 (i.e., the rate at which the output voltage level changes between VHV and 0V during the generation of a binary signal) in accordance with the magnitude of control current ISLEW. Such slew rate controlled level shifters further facilitate the transfer of data between two devices by facilitating, through the adjustment of control current ISLEW, the transmission of the “output” data signals at a slew rate that is supported by the downstream (second) device (e.g., device 42 in FIG. 8).
It has become increasingly desirable to provide electronic systems and devices that exhibit reduced power consumption in order to maximize battery life in portable applications. A problem with conventional slew rate controlled level shifters is that they exhibit quiescent current (leakage) after the “output” signal is held stable at a selected voltage level (i.e., 0V or VHV), thereby undesirably reducing battery life.
What is needed is a slew rate controlled level shifter that minimizes quiescent current after the “output” signal has reached a selected voltage level (i.e., 0V or VHV), thereby undesirably reducing battery life.